Carbonates

The carbonates of alkali metals are all soluble in water and are stable towards heat except Li,CO, which is not only insoluble but also decomposes on heating to lithium oxide. TheDecomposition is made easy because the electrostatic attraction in converting from carbonate to Oxide is considerable. In case of large cation like K in K,CO,, the gain in electrostatic attraction Is relatively much less and the decomposition is difficult.

Sodium carbonate is very important industrial chemical. At temperature below 35.2°C, Na₂CO, crystallizes out from water as Na₂CO,. 10H₂O, which is called washing soda. Above this Temperature it crystallizes as Na₂CO,. H₂O. On standing in air, Na,CO,. 10H₂O slowly loses water And converted to a white powder Na₂CO,.H₂O. The solution of Na,CO, in water is basic due to Hydrolysis of carbonate ion.

Unlike the alkali metal carbonates, the alkaline earth metal carbonates are only very Slightly soluble in water, with the solublity decreasing down the group. They also decompose on Heating and the ease of decomposition decreases down the group.

The ease of decomposition can be related to the size of the metal ion, the smaller the ion, The more is the lattice energy of the resulting oxide and hence higher the stability of the product.

Nitrates

Nitrates of both alkali and alkaline earth metals are soluble in water. Nitrates of Li, Mg, Ca and Ba decompose on heating to give O₂, NO, and the metallic oxide whereas nitrates of Na And K decompose to give different products.

Sulphates

All the alkali metals give sulphates and they are all soluble in water. The solubilities of Sulphates of alkaline earth metals, gradually decrease down the group. BeSO, and MgSO, are Fairly soluble in water. 

1. CaSO, is slightly soluble, while SrSO, and BaSO, are almost insoluble. 

2. Calcium sulphate occurs in nature as gypsum CaSO4.2H₂O. 

3. When it is heated above 100°C, it loses three quarters of its water of crystallization, giving a white powder called’ Plaster of Paris.

COMMERCIAL PREPARATION OF SODIUM BY DOWNS’CELL

Most of sodium metal is produced by the electrolysis of fused sodium chloride. Since the Melting point of sodium chloride is 801°C, some Calcium chloride is added to lower its melting point And to permit the furnace to operate at about 600°C.

In the electrolytic cell, the large block of Graphite at the centre is the anode, above which There is a dome for the collection of chlorine. The Cathode is a circular bar of copper or iron which Surrounds the anode but is separated from it by an Iron screen, which terminated in a gauze. The Arrangement permits the electric current to pass Freely but prevents sodium and chlorine from mixing after they have been set free at the Electrodes, Fig. 2.1.

Sodium metal rises in a special compartment from which it is taken out at intervals. The Cell produces dry chlorine and 99.9 percent pure sodium. The process is carried out at 600°C and It has the following advantages. The metallic fog is not produced. Liquid sodium can easily be collected at 600°C. Material of the cell is not attacked by the products formed during the electrolysis.

PREPARATION OF SODIUM HYDROXIDE BY COMMERCIAL THE DIAPHRAGM CELL

Sodium hydroxide is manufactured on a largeScale by the electrolysis of aqueous solution of Common salt in a diaphragm cell Fig. 2.2 (a). The cell is Made of steel tank. An oblong perforated steel vessel Lined inside with asbestos diaphragm serves as a Cathode. It is provided with a constant level device to Keep the vessel filled to the specified level with brine. A Graphite anode is held within the U shaped diaphragm And it projects into the salt solution. The steam is blown During the process which keeps the electrolyte warm And helps to keep the perforations clear.

The chlorine released at the anode, rises into The dome at the top while hydrogen released at the Cathode, escapes through a pipe. The sodium Hydroxide solution slowly percholates into a catch Basin.

The Fig. 2.2 (b) shows a simplified version of The cell in order to understand the purpose of Diaphragm. When the electrolysis takes place, chlorine Is given off at the anode according to the following Reaction. At the cathode hydrogen is discharged by the reduction of water. The overall result of the above reactions is that the brine loses its chloride ions and the Solution turns increasingly alkaline in cathode compartment. We can face two major problems during the working of the cell.

1. Chlorine produced can react with hydroxide ions in cold giving hypochlorite ions.

2. Hydroxide ions may be attracted towards anode, where they can be discharged releasing Oxygen gas. This oxygen gas may contaminate the chlorine and renders it impure.

The first problem is solved by using asbestos diaphragm. This keeps the two solutions Separate while allowing sodium ions to move towards the cathode. This movement of ions keeps The current flowing through the external circuit.

The second problem is solved keeping the level of brine in anode compartment slightly Higher, this keeps the direction of flow of liquid toward the cathode and thus preventing the Possibility of hydroxides ions to reach the anode.

The solution that flows out of the cathode compartment contains 11% NaOH and 16% NaCl. Evaporation of this solution crystallizes the less soluble NaCl which is filtered off, the Liquid left contains about 50% NaOH and only 1% NaCl as an impurity. For commercial Purposes this small impurity is not important.

ROLE OF GYPSUM IN AGRICULTURE AND INDUSTRY

Role of Gypsum in Agriculture

Bingo Gypsum, a hydrated calcium sulphate, is a mineral that occurs in large deposits Throughout the world. Gypsum is applied to the soil as a source of calcium and sulphur. The calcium supplied By gypsum in fertilizers is of importance in crop production in areas where soils are subject to Extensive leaching.

Sulphur has been recognised as an essential constituent of plants. For centuries, sulphur Compounds had been applied to soils because of their observed beneficial effect on plant growth. Aside from serving as a constituent of protein and various other compounds in plants, sulphur has An influence on chlorophyll development in plant leaves. Although not a constituent ofC  plants deficient in sulphur exhibit a pale green colour.

The root system of several plants have been observed to be greatly enlarged by the Application of sulphur. It has been reported that good crops are produced by the application of Sulphur containing materials such as gypsum.

Role of Gypsum in Industries

When gypsum is heated under carefully controlled conditions, it loses three quarters of Water of crystallization. The resulting product is called Plaster of Paris. Gypsum must not be Heated too strongly as the anhydrous salt is then formed which absorbs water slowly. Such plaster